Redox amplification processes have been described, for example in British Specification Nos. 1,268,126, 1,399,481, 1,403,418 and 1,560,572. In such processes colour materials are developed to produce a silver image (which may contain only small amounts of silver) and then treated with a redox amplifying solution (or a combined developer-amplifier) to form a dye image.
The developer-amplifier solution contains a colour developing agent and an oxidising agent which will oxidise the colour developing agent in the presence of the silver image which acts as a catalyst.
Oxidised colour developer reacts with a colour coupler to form the image dye. The amount of dye formed depends on the time of treatment or the availability of colour coupler and is less dependent on the amount of silver in the image as is the case in conventional colour development processes.
Examples of suitable oxidising agents include peroxy compounds including hydrogen peroxide and compounds which provide hydrogen peroxide, eg addition compounds of hydrogen peroxide; cobalt (III) complexes including cobalt hexammine complexes; and periodates. Mixtures of such compounds can also be used.
A serious problem with developer/amplifier solutions is their stability because they contain both an oxidising agent (eg the peroxide) and a reducing agent (the colour developing agent) which react together spontaneously thus leading to loss of activity in a matter of an hour or two.
In comparison, conventional photographic colour developer solutions suffer loss of activity by aerial oxidation of the colour developing agent. A typical commercial colour developer solution, however, will be stable for a week or two. Both diethylhydroxylamine and hydroxylamine sulphate have been proposed as anti-oxidants for colour developer solutions.
Colour developer solutions for colour negative silver chloride papers do not contain hydroxylamine sulphate because it can act as a black and white developing agent and this severely inhibits dye yield. Instead, diethylhydroxylamine is used because it does not inhibit dye yield.
When using low silver colour papers closely related to currently used silver chloride colour papers to be processed in a redox developer/amplifier it has been found that diethylhydroxylamine works as an effective antioxidant but the bath ceases to be effective after a few hours.
It has now been found that if the pH is raised, the developer/amplifier baths show improved stability on standing.